Inflammatory myofibroblastic tumor of the bladder.

We illustrate a case of an inflammatory myofibroblastic tumor (IMT) involving the bladder in a woman with dysuria and review the literature and differential diagnosis. Inflammatory myofibroblastic tumor, also referred to as pseudosarcomatous myofibroblastic proliferation, is a rare lesion that can arise in the genitourinary system and is characterized by a fascicular arrangement of myofibroblasts with admixed inflammatory cells and slitlike vessels. Urinary bladder IMT can be a diagnostic pitfall because its histologic features (brisk mitoses, invasion into muscularis propria, and prominent nucleoli) can mimic malignancy. The differential diagnosis of urinary bladder IMT includes sarcomatoid carcinoma and leiomyosarcoma. Diagnostic features such as bland nuclear chromatin, ganglion-like cells, pale eosinophilic cytoplasm with long processes, overexpression of anaplastic lymphoma kinase (immunohistochemistry or gene rearrangement studies), and the absence of atypical mitoses help distinguish IMT from its malignant mimics. Current controversies regarding postoperative spindle cell nodule and IMT are discussed.